Polyolefin polymers and polymer blends are known for their versatility and applicability in a wide variety of uses. In particular, many polyolefin polymers, including copolymers of propylene with other α-olefins such as ethylene, are well suited for use in applications requiring good stretchability, elasticity, and strength. Such polymers often comprise a blend of two or more propylene copolymers, and may be manufactured by mechanically blending two or more copolymers, or by in-line reactor blending of the copolymers.
Many polyolefin blends known in the prior art are formed into pellets for intermediate storage purposes before being shaped into articles such as fibers, films, nonwovens, extruded coatings, and molded articles. Some of these compositions, however, are known to exhibit poor pellet stability over extended periods of time, leading to agglomeration of pellets and resulting in pellet batches that a do not flow/pour well, particularly after storage and shipping under hot climate conditions. Further, the typically low melting points of such known polymer blends often lead to flattening or other deformation of polymer pellets during long-term storage, which also negatively affects the ability of the polymer pellets to be free-flowing. While blending such polyolefin copolymers with higher-crystallinity components has been shown to improve stability properties of the polymer pellets, such pellets lose some of their elasticity and still have a tendency to agglomerate during shipping and long-term storage, thus presenting processing issues where free-flowing pellets are required.
As a result, many known polyolefin blend pellets are dusted, such as with a low density polyethylene dust, to prevent agglomeration. The use of such dusts may be problematic for certain applications, however, because they may increase gel levels in films, be unacceptable for use in food contact applications, or create housekeeping issues because of increased dust in the plant environment.
For example, thermoplastic elastomers based on metallocene-polymerized propylene-ethylene copolymers were introduced commercially in 2002-2004. These elastomers have low propylene crystallinity (typically less than 30 J/g) and are truly random with r1r2 of 1 and have high regio- and stereo-propylene defects. While the introduction of both propylene defects and ethylene comonomer in these propylene-ethylene copolymers contribute to a reduction in polymer crystallinity, it also reduces the propylene crystal strengths. As a result, their pellets are soft with Shore A hardness less than 90 and have a strong tendency to agglomerate after pelletization, which often prevents these pellets from being delivered as free-flowing pellets for subsequent processing into fabricated products, particularly under hot weather conditions.
As disclosed in U.S. Pat. No. 7,476,710, U.S. Provisional Application No. 61/305,987 and U.S. Published Application No. 2007/0015877, incorporated by reference herein in their entireties, polypropylene homopolymers or copolymers with moderate to high crystallinity, such as >50 J/g, can be blended in with propylene-ethylene elastomers, typically at levels of 10 to 15 wt %, to obtain acceptable pellet stability.
U.S. Pat. No. 6,268,063 discloses a propylene resin composition featuring good flexibility and transparency without producing sticky feeling, and comprising a polypropylene component and a random copolymer component of propylene and ethylene.
U.S. Published Application No. 2004/0181024 discloses polypropylene-based random copolymer blend compositions which are excellent in processability, stiffness, heat-sealing property, resistance to blocking and surface scratching.
U.S. Pat. No. 7,432,336, incorporated by reference herein in its entirety, discloses propylene-based copolymers and processes for producing them, particularly propylene-ethylene copolymers, which may contain up to 28 wt % ethylene comonomer.
However, none of these prior art efforts has resulted in polymer compositions having satisfactory pellet stability and low cost of production. Thus, there is still a need for improved pellets formed from polymer blends which have long-term pellet stability and are free-flowing without being dusted.